Publication Date

Spring 2024

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Ryan Portner; Manny Gabet; Arran Murch

Abstract

Volcaniclastic deposits formed in shallow and deep marine environments show strikingly different characteristics resulting from the ability of steam to assist in lava fragmentation. Deep marine deposits come from recent lava flows at Axial Seamount, formed around 1,500 m water depth, while shallow marine samples come from Miocene Columbia River Basalt Group (CRBG) lava flows that were emplaced in a shallow marine to subaerial environment and are now exposed on the Oregon coast. Volcaniclastic material from these two environments was compared using granulometry, componentry, and mapping. Lithofacies from the CRBG contained abundant ash and lapilli sized clasts with a high proportion of crystalline clasts, and limited exposures of coherent lava flows. Conversely, volcaniclastic material at Axial Seamount was largely restricted to slopes, with only small collections of fragmented vitriclasts found on and around lava flows. Axial material was also dominantly glassy and coarser than that of the CRBG. This disparity supports an interpretation that steam explosivity played a minor role in forming the volcaniclastic material at Axial Seamount, supported by a lack of ash grain surface textures and grain shapes. Instead, autoclastic fragmentation was interpreted as the dominant fragmentation mechanism there. The characteristics of CRBG volcaniclastic material indicate that the lesser hydrostatic pressure of a shallow marine environment was more conducive to steam explosivity, which occurred in addition to autoclastic fragmentation.

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